The present invention relates to pressure-monitoring stents.
Stents are used in blood vessels to treat the obstruction of blood flow in the cardiovascular system. The use of stents has become a great tool for the treatment of the cardiovascular diseases. Stents include a flexible mesh-like hollow tube that can expand with the help of an angioplastic balloon, thus helping to improve blood flow in cases of occlusion due to plaque accumulation.
Though a stent helps to expand the narrowing effect of the arteries, it also frequently gets “re-covered” by plaque (restenosis) or endothelialization. Patients must be periodically monitored after stent implantation to check for restenosis. It has recently been reported in the literature that re-endothelialization typically takes place within 3-4 months after placing a bare metal stent and within 6 months for drug eluting stent. It has also been reported that 48.8% and 23.1% of the patients experienced restenosis for bare metal stents and drug eluting stents, respectively, from a total of 130 patients in the study. It is estimated that the size of the worldwide vascular stent market was approximately $8 billion in 2008 and $10.5 billion in 2010. Both clinically and economically, there exist great demands in developing an implantable stent that monitors the growth of intravascular tissues without invasive surgery.
Currently the most widely-used monitoring practice is to open a small incision on the patient's body into which a long wire-pressure sensor is inserted. The wire sensor reports pressure differences between two positions across the stent, which indicate the developmental stages of restenosis if any. Note that pressure will be accumulated or decreased respectively in front of or behind the narrowing portion of the vessel, creating a certain amount of pressure difference across the narrowed vessel part. In short, currently patients need to go through incision surgery-based tests every three months for restenosis monitoring.
In order to avoid such invasive procedure, which requires enormous medical costs, expertise, time, and may cause pain to the patient, it would be greatly desirable to develop a stent that can monitor plaque or restenosis development in situ in a non-invasive manner. One possibility is a stent that is capable of monitoring intravascular pressure.
A pressure-monitoring stent is required to provide three specific functions of (1) a mechanical structure to open up the narrowing vessel, (2) pressure monitoring to evaluate any risks of restenosis, and (3) wireless signal transfer from the pressure sensor inside a patient to the external electronic reader. Typically, each function is realized in respective components.
To date, several pressure-monitoring stents have been reported which can be categorized into 3-, 2-, and 1-component systems. While multiple (3 or 2) component systems have demonstrated successful in vivo testing in pigs, they still can impose significant difficulties and dangers during practical surgery, compared to the conventional stent, due to the excessive volume, stiffness, and handling difficulty stemming from the additional components. On the other hand, the 1-component systems provide essentially the same level of surgical risks and procedures as the conventional mechanical stent (i.e. which provides no pressure measurement), while enabling the monitoring of in situ pressure inside the stent. Thus, the 1-component system is clearly more advantageous.
A one-component pressure-monitoring stent system has been developed utilizing magnetoelastic sensors. By utilizing a magnetic material as the stent structure, the stent system, without containing discrete pressure sensors or circuits, is capable of monitoring the plaque deposition through magnetic vibration property changes. When the whole stent is vibrated by the external magnetic field, it produces shifts of the resonance frequency depending on the plaque deposition level that affects the stiffness and mass of the whole stent.
However, the magnetic-vibration-based stent inherently suffers a weak signal that can be easily buried under the mis-alignment or tilting of the monitoring readers and magnetic interference in the measurement environment. This stent also lacks precision, as it provides a lumped output from the whole stent section and does not report the pressure difference across the stent. Further, it does not allow multiple-zone monitoring within the stent interval.
Thus, further improvements in pressure-sensing stents are needed.